1. Field of the Invention
This invention relates to the production of a silicon nitride ceramic by a method which includes a reaction bonding step and a sintering step, carried out during the course of a single, uninterrupted heating step.
2. The Published Art
U.S. Pat. No. 4,356,136, granted Oct. 26, 1982 to Mangels, discloses that a reaction bonded silicon nitride ceramic can be packed in silicon nitride powder and a densification aid, and densified by a two step firing procedure in a nitrogen atmosphere. The first step is in a low pressure nitrogen atmosphere while the second is in a high pressure nitrogen atmosphere. Example 1 of the patent discloses a first step firing in 15 psi. nitrogen to 1875.degree. C. and a second step firing in 300 psi. nitrogen to 1925.degree. C.
U.S. Pat. No. 4,351,787, granted Sep. 28, 1982 to Martinengo et al., discloses that a shape can be pressed from a mixture of silicon particles and a sintering aid, that the shape can be nitrided to produce a reaction bonded shape, and that the reaction bonded shape, after it has been embedded in a protective powder can be densified by firing in a nitrogen atmosphere. In Example 1, the protective powder is composed of Si.sub.3 N.sub.4, BN, MgO and Fe and the firing is to 1800.degree. C., final nitrogen pressure 750 Torr.
U.S. Pat. No. 4,687,655, granted Aug. 18, 1987 to Hunold et al., discloses (Example 6) that ceramics having a silicon nitride content of 99.1 percent, 98 percent of theoretical density, can be produced by cold isostatic pressing of a mixture of silicon powder, yttrium oxide powder and aluminum oxide powder, and subjecting the shapes which are produced to a two-stage firing in a high pressure nitrogen atmosphere. The first stage of the firing was to a maximum temperature of 1270 .degree., nitrogen pressure from 50 to 80 MPa, while the second stage was to a maximum temperature of 1800.degree., nitrogen pressure from 80 to 150 MPa. The patent also states that cycles comparable to the Examples would last from 100 to 140 hours in the case of nitridation with nitrogen or nitrogen/hydrogen mixtures under normal pressure.
It has also been suggested (see, for example, U.S. Pat. No. 4,500,482, granted Feb. 19, 1985) that a mixture of silicon, silicon nitride and an organic binder can be molded to a desired shape, nitrided, and hot isostatically pressed to produce a silicon nitride ceramic and (see, for example, GB PS 1,546,928, May 31, 1979) that a molding can be produced from silicon powder and sintered in an inert atmosphere to 60-75 percent of theoretical density prior to nitriding to form silicon nitride.